Astrophysics > High Energy Astrophysical Phenomena

Abstract: We present general relativistic radiation MHD simulations of super-Eddington
accretion on a $10M_\odot$ black hole. We consider a range of mass accretion
rates, black hole spins, and magnetic field configurations. We compute the
spectra and images of the models as a function of viewing angle, and compare
them with the observed properties of ultraluminous X-ray sources (ULXs). The
models easily produce apparent luminosities in excess of $10^{40}~{\rm
erg\,s^{-1}}$ for pole-on observers. However, the angle-integrated radiative
luminosities rarely exceed $2.5\times10^{39}~{\rm erg\,s^{-1}}$ even for mass
accretion rates of tens of Eddington. The systems are thus radiatively
inefficient, though they are energetically efficient when the energy output in
winds and jets is also counted. The simulated models reproduce the main
empirical types of spectra --- disk-like, supersoft, soft, hard --- observed in
ULXs. The magnetic field configuration, whether MAD ("magnetically arrested
disk") or SANE ("standard and normal evolution"), has a strong effect on the
results. In SANE models, the X-ray spectral hardness is almost independent of
accretion rate, but decreases steeply with increasing inclination. MAD models
with non-spinning black holes produce significantly softer spectra at higher
values of $\dot{M}$, even at low inclinations. MAD models with rapidly spinning
black holes are quite different from all other models. They are radiatively
efficient (efficiency factor $\sim10-20\%$), super-efficient when the
mechanical energy output is also included ($70\%$), and produce hard
blazar-like spectra. In all models, the emission shows strong geometrical
beaming, which disagrees with the more isotropic illumination favored by
observations of ULX bubbles.